System of modulation



Nuv. 29, 1927.

1,650,934 L. w. CHUBB SYSTEM OF MODULAT'ION Filed Aug. 30, 1923 HFG.

WITNESSES: INVENTOR ATTORNEY Patented Nov. 29, 1927..

; STATES PATENT OFFICE.

LEWIS W. CH UBB, OF SWISSVALE, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSEELECTRIC & MANUFACTURING COMYANY, A CORPORATION OF PENNSYLVANIA.

SYSTEM OF MODULATION.

Application filed August 30, 1923. Serial No. 660,126.

This invent-ion relates to systems for sending radio signals and it hasparticular relation to modulating means therefor.

It is an object of this invention to provide means for modulating thesignaling energy without making use of electrical oscillations of otherthan radio frequency.

It is another object of this invention to produce the modulation of theenergy used in sending signal's by means which will not introducedistortions.

It is a further object of this invention to make use of thecharacteristics of the current-capacity curve of a tuned circuit. Such acurve has a well marked peak whose sides are nearly straight lines. Thepractice of this invention includes adjusting the tuned circuit, so thatit normally is working about at the middle of one side of the peak ofthis 7 curve, and producing the modulations by changing the constants ofthe circuit, so that it shall work at points distributed along thestraight part of the curve forming one side of the peak.

It is a further object of this invention to so construct the circuitthat a small change in capacity shall produce a maximum change incurrent. More specifically stated, this object is accomplished by makingthe coupling between the tuned circuit and the energizing circuit veryloose, so that the peak of the current-capacity curve will be verynarrow, and then tuning, as already described, to the middle of oneslope of the peak.

It is a still further object of this -invention to maintain the grid ofa thermionic tube at such potential that a minimum amount of power willbe required in the tuned circuit controlling the grid.

Other objects of the invention and details of construction will beapparent from the following description and the accompanying drawings,wherein,

' The single figure is a diagrammatic illustration of circuits andapparatus embodying my invention.

In this figure, 1 is a generator of high frequency current, which hasbeen indicated by the letters HFG on the drawing. No particular type ofgenerator need be illus' trated or described because the invention isequally applicable to systems supplied by generators of many types.

A loose cou ling 2 includes a primary coil 3 supplied y the highfrequency generator and a secondary coil 4 which is part of a circuitincluding an adjustable condenser 5 and a current measuring instrument6. The coupling 2 is made loose in order that the current-capacity curvefor the secondary circuit shall have a very sharp and narrow peak. Thiscircuit also includes a variable condenser 7, in parallel with theinductor 4 and condenser 5. As illustrated, this condenser comprises astationary plate 8 and a movable plate 9, insulated from each other. Theplate 9 forms one wall of a chamber into which the mouthpiece 11 willconduct sound. The sound waves, by causing the diaphragm 9 to vibrate,will change the capacity of the condenser .7, so that the total capacityin parallel with the inductor l is varied.

One terminal of the circuit including this capacitance and inductance isconnected to the filament 12 of a thermionic device 13 and the otherterminal is connected to the grid 14 of the same vacuum tube. Theconnection to the grid preferably includes a C-battery 15 formaintaining the grid at a negative potential. The anode 16 is connectedto the usual output circuit. Further amplification of the output isprovided by a circuit 17, which, as illustrated, includes two thermionicdevices arranged in parallel. The invention 1s, however, equallyapplicable to a system including any form of amplifying device. Theamplifying circuit just mentioned is coupled to the output circuit ofthe vacuum tube 13 by means of a transformer 18. The output circuit ofthe amplifier 17 is coupled, as shown at 19, to the antenna 21, fromwhich the energy is radiated.

In the operation of the device, the adjustable condenser 5 ismanipulated until the.

reading of the current-measuring device 6 is maxlmum and then changed ineither direction until the reading of the instrument is approximatelyhalf of the maximum reading. If a curve be plotted using values of thetotal capacity of the condensers 5'and 7 combined, as abscissa andcurrent in the circuit including inductor4 and said condensers, asordinates, its form will be closely similar to the familiarcurrent-frequency curve. That is, it will have a peak with rather steepand nearly straight sides. The circuit including the inductor 4 andcondenser 5 will after thisadjustment be operating at a pointapproximately'half way up one 0 side of the peak of the currentcapacitycurve.

The capacity of the variable condenser 7 is then changed. This can bedone by speaking into the mouthpiece 11. It will however, be obviousthat any other metho of varying this capacity would be equallyefiective, for example, the movable element 9 of the condenser 7 mightbe connected to a phonograph mechanism so that the record would causethe element 9 to vibrate.

Each change in the capacity of the condenser 7 changes the point on thecurrentcapacity curve at which the circuit through inductor 4 isworking. It thus changes the energy delivered to the grid 14:. Thedisplacement, from mid-position on one side of the peak of the curve, isnot great enough to reach beyond the practically straight part of thecurve, consequently the changes in amplitude of the potential vibrationon the grid 14 will be strictly proportional to the changes in capacityof the condenser 7 and, therefore, strictly proportional to the soundvibrations or other disturbances displacing the movable element 9. Thebattery 15 has its potential so chosen that the position of the pointrepresenting the average potential of the grid 14 is in the middle ofthe straight part of the characteristic curve of the tube 13. Thechanges in the potential of this grid do not extend beyond the straightpart of the curve. No distortion, therefore, is introduced by theamplifying action of the tube 13. Thus, the whole system. from thecondenser 7 to the antenna 21, includes nothing that will cause adeparture from this strict proportionality, consequently the energyradiated from the antenna 21 will represent, without distortion, thesound spoken into the mouthpiece 11.

Although, I have described and illustrated but a single embodiment of myinvention, it will be apparent to those skilled in the art that manyvariations may be made without departing from the spirit thereof. and I,therefore, do not intend my invention to be limited except as isnecessitated by the prior art or indicated by the claims.

I claim as my invention:

1. In a radio sending system, a circuit ineluding a source of highfrequency energy, a second circuit loosely coupled thereto and includingan inductor and a tuning condens er in parallel, a sound responsivecondenser in parallel to said tuning condenser, the capacity of saidtuning condenser and the average capacity of said sound responsivecondenser together being so related to the inductance of said inductorthat the circuit is not resonant to the frequency of said source, but isso nearly resonant thereto that the variations in current resulting fromthe changes in capacity of the sound responsive condenser bear a linearrelation to said changes.

2. -In a radio transmitting system, a source of high frequencyoscillations, a thermionic amplifier having an input circuit coupled tosaid source. means for tuning said input circuit, signal responsivemeans for causing the period of said circuit to approach and rccede fromresonance with said source, and means whereby a linear relationship ismaintained between the signal and the output of the amplifier.

3. In a radio transmitting system, a source of high frequencyoscillations, a thermionic amplifier having an input circuit coupled tosaid source, means normally maintaining the tuning of said input circuitslightly out of resonance with said source, signal responsive meansassociated with said input circuit for slightly altering its tuning, andmeans whereby theoutput of said amplifier is maintained directlyproportional to the signal.

4. In a radio transmitting system, a source of high frequencyoscillations, a thermionic amplifier having an input circuit coupled tothe grid and filament thereof. signal responsive means for slightlyaltering the amount of current flowing in said input circuit by reasonof its coupling to said source, and means for maintaining the output ofsaid thermionic amplifier directly proportionalto the signal.

In testimony whereof. I have hereunto subscribed my name this 23rd dayof August, 1923.

LEWIS IV. CHUBB.

